As wireless portable communication devices, such as cellular telephones, are made smaller, corresponding components including antennas used for those devices are also made smaller, and/or are sometimes at least partially integrated with other components. As a result, a printed circuit board (“PCB”), which is populated with electronic and mechanical components, is effectively used as part of radiating antenna elements of the wireless portable communication device. Radio frequency (“RF”) current flows on the PCB and the PCB acts as an antenna. For cellular telephones operating in lower frequency band such as Global System for Mobile Communications (“GSM”), which covers the frequency band from about 880 MHz to 960 MHz, and Advanced Mobile Phone System (“AMPS”), which covers the frequency band from about 824 MHz to 894 MHz, the effect of the PCB radiation is more apparent compared to cellular telephones operating in higher frequency band such as Personal Communications Services (“PCS”), which covers the frequency band from about 1850 MHz to 1990 MHz. Because the size of an antenna is typically made to have an electrical length corresponding to the wavelength of the frequency used, the size of the antenna is generally larger for a lower frequency application.
By using a PCB of a cellular telephone as a radiating element, the radiating efficiency of the cellular telephone may be easily degraded due to the PCB being in close proximity to a user's body. For example, when a cellular telephone is used, it is typically held in a user's hand, which essentially covers one side of the PCB, and the other side of the PCB is held against the user's face. When the cellular telephone is carried in a user's pocket or is carried by a belt-clip, one side of the PCB faces the user's body. This presence of the user's body in close proximity to the PCB, which is being used as a radiating element, may significantly affect the radiation efficiency of the cellular telephone.
A cellular telephone may use a variety of types of antennas, such as a helical antenna and an internal antenna. The helical antenna may be viewed as a dipole-like structure comprising the antenna as a quarter-wave radiator and the PCB as another quarter-wave radiator. The internal antenna may be viewed as a matching network to the PCB for the 824-960 MHz bands of operation. Compared to the PCB, the antenna itself is generally much smaller in volume, and therefore contains more concentrated radiation energy than the PCB. To reduce the degradation in efficiency due to the presence of the user's body in close proximity, the antenna is typically located in the cellular telephone where it is kept away from the body of the user. However, the PCB, having RF currents flowing and emitting radiation, is still kept next the user's body, and the radiation efficiency of the cellular telephone is still considerably susceptible to the proximity of the user's body to the PCB.